8 Macfarlane. — Contributions to the History of 



heads : (i) Leaf closure ; (2) Leaf structure; (3) Leaf secre- 

 tion ; (4) Leaf opening ; while the keynote to the whole is 

 given in the words used by Burdon Sanderson in his lecture 

 before the Royal Institution. 1 "We have to do here not merely 

 with contractility, but with irrito-contractility." 



I. Leaf Closure. 



(a) By Mechanical Stimuli. 



Except for the partial limitation contained in the statement 

 by Sanderson and Page above given, all observers have hith- 

 erto asserted that the leaf closes after a single touch. Thus 

 Darwin says, 2 "These filaments are remarkable from their ex- 

 treme sensitiveness to touch, as shown not by their own move- 

 ment, but by that of the lobes." Sachs 3 says, " any ungentle 

 touch of one of these bristles effects an instantaneous closure of 

 the two halves of the leaf." Detmer* says, "Werden die Fila- 

 mente, die auf der oberseite des Dionaea blattes vorhanden 

 sind, beriihrt, Z. B. mit einem kleinen Holzstiickchen, so 

 schliesst sich das Blatt sofort." Drude, Batalin, Munk and 

 others, speak in similar terms. Now we can only explain this 

 practically universal consensus of opinion to be due to obser- 

 vers having supposed that after the first touch of a hair with 

 non-closure, they imagined that the leaf was torpid, or that 

 they had missed touching one, and repeated the touch with 

 desired result. In any case it can now be asserted that under 

 such conditions as the plant is normally exposed to, two touches 

 are needed to cause closure. It matters not whether these 

 are communicated to the same hair, or to distinct hairs on the 

 same half, or on opposite halves of the leaf. 



But though no apparent movement ensues after the first 

 touch, if a hair on the leaf of a vigorous plant be once touched 

 during warm weather, by steady attention one can readily see 

 that a peculiar rhythmic wave motion traverses the leaf halves 

 in line with the length of the blade for about five seconds 



1 Nature; Vol. X, 1874, p. 127. 



2 Insectivorous Plants, p. 287. 



3 Physiology of Plants ; Eng. ed. 1887, p. 376. 



4 Pflanzen physiologische Practicum ; 1888, p. 64. 



